1. Field of the Invention
The present invention relates to a vibration detecting device used for detecting a vibration applied to a vehicle, and a vehicular road simulator employing the vibration detecting device.
2. Description of Background Art
There has been developed a vehicular road simulator for performing a vibration test on a vehicle without actually driving the vehicle by the steps of actually running a vehicle such as a motorcycle, an automatic four-wheeled vehicle or the like; collecting the vibration waveform applied to the vehicle as an actual vibration waveform data; and applying, on the basis of the collected actual vibration waveform data, the same vibration as that applied to the actual traveling vehicle to the vehicle mounted on a testing base by a hydraulic actuator.
A vehicle vibration test is performed as follows: First, a vibration applied to a vehicle traveling along a road surface is detected by an accelerometer, and the detected acceleration is recorded in a data recorder. In the road simulation by a vehicle simulator, on the basis of the vibration data thus collected, a vibration is applied to the vehicle mounted on a testing base by way of the axle shaft or wheels of the vehicle by an actuator. As for the activation by the actuator in this vehicle vibration test, a transfer function of a system including the actuator, vehicle and accelerometer is obtained by repeating trial activation; an activation control signal is obtained on the basis of the transfer function and the collected actual vibration waveform; and the same vibration as that applied to the actually traveling vehicle is applied to the vehicle by the actuator while controlling the activation on the basis of the activation control signal. In addition, at the times of both the actual running and the simulation testing activation, the same accelerometer is used.
In the above-described load simulation by the vehicle road simulator, the reproduction of a vertical vibration applied to a traveling vehicle is generally performed by recording the acceleration of the traveling vehicle by an accelerometer attached near the axle shaft of the vehicle, and applying, to the vehicle mounted on a testing base, the same vibration as that with the vibration waveform thus recorded.
On the other hand, it may be considered that a linear voltage differential transformer (LVDT) or a velocity meter is used for detecting the vibration of a vehicle. However, it has a structure of two parts, that is, a coil section and core section, and is not easy to be mounted on the vehicle. Accordingly, it is not used for detecting the vibration of a vehicle. An optical displacement meter has a structure of one part, but is low in the sensitivity for a vibration with high frequency. Accordingly, it is not used for detecting the vibration of a vehicle. In contrast, an accelerometer is suitable for detecting the vibration of a vehicle, because it is compact and lightweight, and further it has an integrated structure and is simply mounted on a vehicle.
For the above-described reason, the accelerometer is used for detecting the vibration of a vehicle in the vehicle road simulator, and the acceleration applied to a vehicle is detected by the accelerometer, and is collected. However, in the detection of the acceleration applied to a vehicle, the accelerometer is required to withstand accelerations having an upper limit frequency of 100 Hz or more and an acceleration level of 50 (G) or more.
However, an accelerometer capable of withstanding accelerations having an upper limit frequency of 100 Hz or more and an acceleration level of 50 (G) or more has a disadvantage that it is low in the sensitivity for vibrations having low frequencies. Thus, it is difficult to reproduce frequency components of 2 to 3 Hz or less in a vehicle load simulator upon test activation. This is true, in particular, for a motorcycle, which has a longer suspension than that of an automatic four-wheeled vehicle. Accordingly, a vehicle load simulator for a motorcycle requires the detection of low frequency components of vibration. Even an automatic four-wheeled vehicle has main frequencies of rolling or pitching of the body in the range from 1 to 2 Hz. Accordingly, a vehicle road simulator for an automatic four wheeled vehicle also requires the detection of low frequency components of vibration.
In the detection of vibrations of a vehicle, there is an inconvenience in that an acceleration detecting device selected for detecting the maximum acceleration cannot detect low frequency components of vibration. To cope with this inconvenience, as exemplified in unexamined Japanese Patent Publication No. Hei-3-295437, an accelerometer having a good sensitivity for low frequency components of vibration is provided in addition to an accelerometer suitable for the detection of the maximum acceleration. In this case, for example, the accelerometer suitable for low frequency components of vibration is mounted on a measurement portion of the body where low frequency components of vibration are generated. Thus, there is a disadvantage in preparing two kinds of accelerometers.
Moreover, an accelerometer having a good sensitivity for low frequency components of vibration is expensive, and requires careful handling because it is sensitive to shock and easily broken. In the case of traveling along a rough pavement to gather data for a road simulation by a vehicle load simulator and of testing the activation of such a simulator, an accelerometer for detecting low frequency components of vibration cannot withstand accelerations exceeding an allowable value and thus is often broken. Accordingly, an accelerometer having good sensitivity for low frequency components of vibration is difficult to be practically used in a vehicle load simulation.
Moreover, in the case of detecting the acceleration by use of two kinds of accelerometers mounted on a vehicle load simulator, there occurs a disadvantage in that the provision of the two accelerometers is accompanied by an increase in the number of amplifiers for amplifying the output of the accelerometers and the channels of the collecting devices for collecting acceleration data.
Additionally, the provision of two accelerometers has the following disadvantages. In the case of conversion into digital data, there is an increase in the number of A/D converters, D/A converters for driving an actuator, correcting circuits for correcting the D/A conversion output on the basis of a transfer function, and the like. Moreover, in the case where the correction on the basis of a transfer function is required to be performed by a computer, the road on calculation and the data area are increased linearly with the number of channels, thereby deteriorating the response of a computer control system. Further, the programming becomes complex.